1. Field of the Invention
The present invention relates to a light wave distance measuring apparatus for measuring a distance to an object to be measured by utilizing an amplitude-modulated light wave.
2. Related Background Art
As for a conventional light wave distance measuring apparatus, there are known one employing a time-of-flight (TOF) system, and one employing an amplitude modulation (AM) system.
The light wave distance measuring apparatus of the TOF system operates such that a distance measuring light emitted in pulse-like shape is applied to an object to be measured, and the distance measuring light reflected by the object to be measured is received to observe a distance to the object to be measured based on a time delay from a time point of the application of the distance measuring light to a time point of the reception of the distance measuring light (refer to JP 07-63853 A). The light wave distance measuring apparatus of this system requires a broadband amplifier and a very high speed arithmetic operation circuit, because, when the high precision is required for the measurement, an extremely short time must be measured. Thus, the light wave distance measuring apparatus of this system is difficult to be manufactured in terms of the technique as well, and the components or parts used therein become expensive. On the other hand, the light wave distance measuring apparatus of the AM system can be constructed with inexpensive components or parts, because the distance can be measured with high precision using a relative low frequency of several tens of MHz. For this reason, the AM system has been utilized in the light wave distance measuring apparatus requiring the wide use in many cases. The principles of the measurement in the AM system will hereinafter be described with reference to FIG. 7.
A distance measuring light which is amplitude-modulated with a signal having a frequency f is applied from a light source portion 70 such as a laser to an object 80 of measurement, and the distance measuring light reflected by the object 80 of measurement is received by a light receiving portion 90 such as an avalanche photodiode (APD). At this time, the received distance measuring light has a phase difference ΔΦ corresponding to a distance L to the object to be measured. Then, when the phase difference ΔΦ between the distance measuring light upon its application and the distance measuring light upon its reception is measured, the distance L can be obtained from Equation 1:
                    L        =                              C                          2              ⁢              f                                ×                                    Δ              ⁢                                                          ⁢              Φ                                      2              ⁢                                                          ⁢              π                                                          Equation        ⁢                                  ⁢        1            where C is a light velocity.
Note that in the actual measurement, the frequency of the signal with which the distance measuring light is amplitude-modulated is changed by two or more kinds. The reason for adopting such a process resides in that it is avoided that the measurement becomes impossible for a distance equal to or longer than C/2f because when the distance L becomes a multiple of C/2f in Equation 1, the phase difference ΔΦ becomes zero (refer to JP 2002-90455 A).
However, in the conventional light wave distance measuring apparatus of the AM system, in order to directly detect the phase difference between the distance measuring light upon its application and the distance measuring light upon its reception from the distance measuring light, the distance measuring light must be A/D-converted using a sampling signal having a frequency which is several times as high as that of the distance measuring light. For this reason, an expensive A/D converter is required which is responsible to the frequency of the sampling signal.